Numerous scaffolds have been developed for tissue engineering applications. These scaffolds provide a template on which cells can migrate, divide, secrete new matrix and differentiate. Typical tissue engineering scaffolds are porous and can be categorized as having pores on either a micrometer scale, i.e. microporous, or a nanometer scale, i.e. nanoporous. Scaffolds having pores on a micrometer scale, or having average pore diameter of about 10 to 1000 microns, are composed of a variety of biocompatible materials including metals, ceramics and polymers. Such scaffolds include solid-cast structures, open-pore foams, felts, meshes, nonwovens, woven and knitted constructs. The mechanical and conformational properties can be chosen by composition of the material and the design of the scaffold. Desirable mechanical properties include the ability to be sutured in place and good handling strength.
Composition, design and construction of the scaffold are also important to how tissue responds to the scaffold. The scaffold can be shaped to maximize surface area, to allow adequate diffusion of nutrients and growth factors to cells present in or growing into it. For example, the maximum distance over which adequate diffusion through densely packed cells can occur is in the range of about 100 to 300 microns, under conditions similar to those that occur in the body, wherein nutrients and oxygen diffuse from blood vessels moving into the surrounding tissue. Taking these parameters into consideration, one of skill in the art would configure a scaffold having pores on a micrometer scale as having sufficient surface area for the cells to be nourished by diffusion until new blood vessels interdigitate the implanted scaffold.
Scaffolds having pores on a nanometer scale, e.g. having average pore diameter of about 10 nanometers to 1 micron, are often composed of hydrogels. A hydrogel is a substance formed when a natural or synthetic organic polymer is cross-linked via covalent, ionic or hydrogen bonds to create a three-dimensional open-lattice structure, which entraps water molecules and forms a gel. Examples of materials that can be used to form a hydrogel include polyamides, methylcellulose, collagen, extracellular matrix (ECM), polysaccharides such as alginate, polyphosphazines, polyacrylates which are crosslinked tonically, high molecular weight poly(oxyalkylene ether) block copolymers such as those sold under the tradename PLURONCIS (BASF Corp., Mount Olive, N.J.), nonionic polymerized alkylene oxide compounds such as those sold under the tradename TETRONCIS (BASF Corp., Mount Olive, N.J.), or polyethylene oxide-polypropylene glycol block copolymers which are crosslinked by temperature or pH, respectively.
Hydrogels provide conformable, malleable, or injectable materials for administering cells into a tissue. They do not, however, have mechanical integrity. Synthetic hydrogels can be sterilized and do not have the risk of associated infectious agents. However, most synthetic hydrogels do not mimic the extracellular matrix and therefore do not direct cellular ingrowth or function. Hydrogels of natural extracellular matrix are biocompatible and can mimic the native cellular environment. However, natural hydrogels, unless made from autologous material, may elicit an immune response and may have associated infectious agents. Natural hydrogels, such as EHS mouse sarcoma basement membrane, or fibrin, have a fiber diameter of about 5 to about 10 nanometers, water content of about 80 to about 97 weight percent and average pore diameter of about 50 to about 400 nanometers.
It would be advantageous to provide a tissue engineering scaffold that provides benefits of both microporous and nanoporous scaffolds while avoiding problems associated with certain hydrogel scaffolds as noted above. The present invention provides such an advantage by selectively combining a microporous scaffold and a nanoporous scaffold, resulting in a tissue engineering scaffold possessing mechanical properties necessary for use and unexpected enhanced tissue ingrowth and vascularization.